Topographical models of geographical areas may be used for many applications. For example, topographical models may be used in flight simulators and for planning military missions. Furthermore, topographical models of man-made structures (e.g., cities) may be helpful in applications such as cellular antenna placement, urban planning, disaster preparedness and analysis, and mapping, for example.
Various types of topographical models are presently being used. One common topographical model is the digital elevation model (DEM) or digital surface model (DSM). A DEM is a sampled matrix representation of a geographical area which may be generated in an automated fashion by a computer. In a DEM, coordinate points are made to correspond with a height or elevation value. A tiled triangulated irregular network (T-TIN) is another type of geospatial model. As would be appreciated by the skilled artisan, a triangulated irregular network may include a surface representation derived from irregularly spaced sample points and break line features. The T-TIN data set includes topological relationships between points and their neighboring triangles. Each sample point has an x, y coordinate and a surface, or z-value. These points are connected by edges to form a set of non-overlapping triangles used to represent the surface. Tins are also called irregular triangular mesh or irregular triangular surface models.
Conventional DEMs are typically used for modeling terrain where the transitions between different elevations (e.g., valleys, mountains, etc.) are generally smooth from one to a next. That is, DEMs typically model terrain at spacings of 0-30 meters presently and as a plurality of curved surfaces and any discontinuities there between are thus “smoothed” over. Thus, in a typical DEM distinct objects may not be present on the terrain.
One particularly advantageous 3D site modeling product is RealSite® from the present Assignee Harris Corp. RealSite® may be used to register overlapping images of a geographical area of interest, and extract high resolution DEMs using stereo and nadir view techniques. RealSite® provides a semi-automated process for making three-dimensional (3D) topographical models of geographical areas, including cities that have accurate textures and structure boundaries. Moreover, RealSite® models are geospatially accurate. That is, the location of any given point within the model corresponds to an actual location in the geographical area with very high accuracy. The data used to generate RealSite® models may include aerial and satellite photography, electro-optical, infrared, and Light Detection and Ranging (LIDAR), for example.
Another advantageous approach for generating 3D site models is set forth in U.S. Pat. No. 6,654,690 to Rahmes et al., which is also assigned to the present Assignee and is hereby incorporated herein in its entirety by reference. This patent discloses an automated method for making a topographical model of an area including terrain and buildings thereon based upon randomly spaced data of elevation versus position. The method includes processing the randomly spaced data to generate gridded data of elevation versus position conforming to a predetermined position grid, processing the gridded data to distinguish building data from terrain data, and performing polygon extraction for the building data to make the topographical model of the area including terrain and buildings thereon.
One potentially challenging aspect of generating geospatial models such as DEMS is that high resolutions (i.e., data point or post spacing of ≦1 m) are becoming the norm for terrain representation and an important part of the process to create 3D city models (e.g. Virtual Earth), emergency planning efforts (e.g. flood plane studies), battle damage assessment and/or city planning efforts (e.g. skyline prediction), for example. As the density of data points in high resolution DEMS (HRDEMs) increases, so too does the volume of data generated for such models. The size of these models can be extremely burdensome to even the most powerful geospatial data processing computers in some applications.
An integrated approach may be helpful to create DEMs or DSMs using available data, e.g. from multiple sources. For example, the approach should be capable of using multiple overlapping and non-overlapping stereo image pairs, including edge data from the images and/or area correlation data, as well as known ground truth information regarding roads and/or water boundaries, for example, and image segmentation data. The approach should preferably be capable of using relatively small correlation patches, e.g. 3×3 patches.
Referring to the schematic diagram of FIG. 1, current approaches may include the extraction of patches P from left and right images 100, 102, correlation of patches to form a correlation surface 104 and finding the peak thereof. The peak location is analyzed to compute the height and location of the elevation post, and the process may be repeated for multiple points on an initial post grid. The DEM is generated and then corrected with ad-hoc techniques to incorporate additional data. However, each post height is computed from local data only, the approach is only typically applicable to a single stereo pair of images, and the combining of DEMs (composite DEMs) and additional truth data (e.g. roads, lakes, etc.) using ad-hoc techniques may lead to complex and error-prone results. Moreover, the larger correlation patches (e.g. 16×16) may blur details of the DEM and limit resolution, and elevations may not be produced at desired post locations and are typically interpolated to get height at the desired locations.